Should those checkboxes be replaced with a Special:Listusers like dropdown? Or should the checkboxes be left as is and and a new dropdown be added there without removing the checkboxes? Personally, I think the former option is better.

I'm considering possible cases: we had a way before to choose multiple
combinations with the checkboxes. I know some requests I can have to make
can be to select multiple roles at the same time. But I can do several
requests instead of one if that's too complicated to have a multiple-select
dropdown. :)

The very best would have been to have filters (all sysops or bureaucrats
but not sysops who are bureaucrats), but I think that requires more work.

I think it would be useful to have the ability to select multiple values from the input (e.g. Administrators (sysop) and stewards). However, currently, we don't have a nice widget for this, the only one would be a CapsuleMultiselectWidget (in OOJsUI), which, unfortunately, lacks a php only widget (so it works with JavaScript only, and therefore can't be used for forms, that are (and need to be) working without JavaScript. After this problem is resolved, we can create a HTMLForm widget for that and use it on Special:ActiveUsers. For the meantime, I think that the current state is a big improvement and can be used until we've a generalised multi-select widget (there are some open tasks for that, too: https://phabricator.wikimedia.org/search/query/erXUJ6Y5J5vT/#R).

@matmarex Yeah, multiselect would allow rendering of "Active users" for either all users, or for one or more specified groups.

However the old functionality wasn't whitelisting two groups, it was excluding two groups. Specifically with the purpose to render a list of all active users that weren't bots or administrators.

Even if we ignore the use case of "Hiding both bots and administrators". The use case of "Hide bots" seems quite useful and would not be satisfied by a multiselect. Even if a user would try to work around it by selecting all other groups, that would be terrible UX, but also not work since a bot may be in multiple user groups. And the resulting permalink would break whenever a new group is created.

Is it important to completely eliminate checkboxes?. We can add them in the group menu so the users can see what they have checked .They can again uncheck some options after which the result gets displayed immediately

"nobots" feature is now absent. As pointed out in T116354#2730127 this cannot be worked around.

Okay, I agree this was a mistake. The patch above brings this back.

Labels have a blank space to the left of them that isn't there initially.

This looks like a new Chrome bug regarding the display of <legend> elements, because I can't reproduce this in any other browsers, and it definitely was not happening when I was merging that patch. Filed as T151061.

Checkboxes are visible at first but are replaced with a dropdown menu after 1-2 seconds or so when using www.mediawiki.org (Chrome latest, MacBook Pro, Fibre, unthrottled)

Yes, this is known and was discussed on the patch. I think it's a very small issue, and resolving it would be a lot of work (we'd have to rewrite all the JS code that renders the dropdown into PHP, and render both the functional checkboxes and this non-functional fake in HTML output).